The present invention relates to a method for monitoring a dosage pump intended for pumping a concentrate to be diluted in water. More particularly, the present invention relates to a monitoring system for a dosage pump for pumping a concentrate fluid at a very low flow rate, preferably in connection with a dialysis machine.
European Patent Application No. 278,100 describes a dialysis machine in which the present invention may be utilized. That dialysis machine comprises a preparation unit for a dialysis fluid in which preparation of the dialysis fluid takes place on-line, starting from concentrates in powder form located in separate cartridges.
Normally, a dialysis machine comprises two portions, a first blood handling portion for feedings blood from a patient through an extracorporeal fluid circuit comprising a dialyzer, and a second dialysis fluid handling portion for preparing a dialysis fluid and transporting it through the dialyzer, and then to a drain. The dialyzer comprises a semi-permeable membrane separating the dialyzer into a blood containing portion and a dialysis fluid containing portion. Transport of molecules, ions, substances and water takes place across the membrane for conditioning the blood to replace the function of the kidneys.
The dialysis fluid normally has a composition which substantially matches that of the patient""s blood plasma, with certain modifications. In addition to water, a dialysis fluid normally comprises the following substances in ionic form: sodium, bicarbonate, potassium, magnesium, calcium, chloride and acetate. The pH value of the fluid is adjusted to between about 7.1 and 7.4. In addition, the fluid may comprise glucose and other substances.
Two ions are present in large quantities in the dialysis fluid, namely sodium and bicarbonate.
European Patent Application No. 278,100 describes the preparation of a dialysis fluid in which these two ions are obtained on-line from powder cartridges containing sodium bicarbonate and sodium chloride, respectively, in the form of a dry powder or granules. Water is passed through the cartridges and substantially saturated fluids of sodium bicarbonate and sodium chloride, respectively, exit the cartridges. Two dosage pumps ensure that the correct quantity of concentrate fluid is fed to a main conduit comprising clean water obtained from a reverse osmosis unit.
The dialysis fluid normally comprises about 35 mmol/l of bicarbonate and about 140 mmol/l of sodium. In total, about 120 liters of dialysis fluid is consumed during one treatment, which normally lasts for four hours and takes place three times a week.
Furthermore, the dialysis fluid contains magnesium, potassium, calcium, acetic acid and glucose in suitable quantities. In the dialysis machine according to European Patent Application No. 278,100, these other components are obtained from an ionic bag. Since these substances have a relatively low concentration in the prepared dialysis fluid, the contents of the ionic bag can be very concentrated, in the ratio of about 1:200 to 1:500, whereby the volume of the bag is small, about xc2xd liter.
The dosing of the contents of the ionic bag is performed using a dosage pump. The dosage pump feeds the contents of the ionic bag to the main conduit in the dialysis machine with a flow rate of about 1 ml/min.
A dialysis machine further comprises a supervisory system which supervises or monitors vital operations of the dialysis machine. A malfunction of such vital operations could result in the patient not obtaining adequate treatment, becoming ill, or being harmed or even dying.
One operation which must be monitored is the dosage pump of the ionic bag. Too high a dosage of the contents in the ionic bag could lead to heart failure, while too low a dosage could lead to other symptoms.
It is not simple to monitor such a low flow rate as that which passes from the ionic bag; i.e., the order of about 1 ml/min. A large deviation must be able to be noted quickly enough for suitable corrective measures to be undertaken, at least within one minute, and preferably within ten seconds. The accuracy must be high and at least within the range of about +/xe2x88x925%.
The contents of the ionic bag comprise salts having a high ionic strength. Mechanical flow measurement devices run the risk of jamming if salt crystals are precipitated, for which there is a great risk.
It is previously known to measure such small flows using thermal flow sensors (see, for example, German Patent Application No. 4,127,675). These sensors are, however, greatly influenced by a change in the ambient temperature, and false alarms may easily be emitted. A dialysis machine must operate equally well at temperatures of about 20xc2x0 C., as well as at ambient temperatures of 35xc2x0 C., which may be the case in certain countries. In addition, large temperature differences and temperature changes arise internally in a dialysis machine, for example during and shortly after heat sterilisation, which may also lead to problems. In certain cases, a thermal flow detector must be calibrated for different types of fluids because of different densities and heat capacities dependent on the concentration of the constituent substances.
One object of the present invention is to provide a system and a method for monitoring a dosage pump intended for low flow rates, in the order of about 1 ml/min, which is accurate and able to trigger an alarm signal within a reasonable time.
A further object of the present invention is to provide a monitoring system for a dosage pump for low flow rates which is sturdy, and scarcely affected by the surroundings, such as ambient temperatures.
In accordance with the present invention, these and other objects have now been realized by the invention of apparatus for monitoring the flow of a fluid through a dosage pump having a suction stroke for drawing the fluid into the dosage pump from a source of the fluid and a discharge stroke for discharging the fluid from the dosage pump, the apparatus comprising an auxiliary pump disposed between the fluid source and the dosage pump, a slave chamber disposed between the auxiliary pump and the dosage pump, the slave chamber including a level detector for detecting a first predetermined level of the fluid in the slave chamber and emitting a signal when the level of the fluid in the slave chamber is below the first predetermined level, and control means for activating the auxiliary pump after the level detector emits the signal whereby the slave chamber is refilled with the fluid by the auxiliary pump after the suction stroke of the dosage pump has drawn the fluid into the dosage pump and caused the level indicator to emit the signal. In a preferred embodiment, the dosage pump is incorporated in a dialyzer.
In accordance with one embodiment of the apparatus of the present invention, the control means is adapted to activate the auxiliary pump to refill the slave chamber to a second predetermined level above the first predetermined level of the fluid in the slave chamber.
In accordance with another embodiment of the apparatus of the present invention, the apparatus includes regulating means for regulating the dosage pump whereby the suction stroke is carried out at a first speed and the discharge stroke is carried out at a second speed, the first speed being substantially greater than the second speed and the second speed providing a substantially constant flow rate.
In accordance with another embodiment of the apparatus of the present invention, the auxiliary pump includes measuring means for measuring the volume of the fluid pumped by the auxiliary pump during each cycle thereof, the control means including time measuring means for measuring the time between each cycle of the auxiliary pump and calculating means for calculating the flow of the fluid through the dosage pump based on the ratio between the volume of the fluid measured by the measuring means and the time between each of the cycles of the auxiliary pump measured by the time measuring means.
In accordance with another embodiment of the apparatus of the present invention, the auxiliary pump comprises a second dosage pump having a predetermined volume per cycle or portion thereof, and wherein the slave chamber includes a side wall and an outlet for the second dosage pump, the inlet being disposed adjacent to the side wall of the slave chamber.
In accordance with the present invention, a method has also been devised for monitoring the flow of a fluid through a dosage pump having a suction stroke for drawing the fluid into the dosage pump from a source of the fluid and a discharge stroke for discharging the fluid from the dosage pump, an auxiliary pump disposed between the source of the fluid and the dosage pump, and a slave chamber disposed between the auxiliary pump and the dosage pump, the method comprising detecting the level of the fluid in the slave chamber and emitting a signal when the level is below a first predetermined level in the slave chamber, and actuating the auxiliary pump after emitting the signal whereby the level of the fluid in the slave chamber is increased above a second predetermined level by the discharge stroke of the dosage pump. In a preferred embodiment, the dosage pump is incorporated in a dialyzer.
In accordance with one embodiment of the method of the present invention, the second predetermined level is greater than the first predetermined level, whereby the slave chamber is topped up with a predetermined hysteresis value above the first predetermined level.
In accordance with another embodiment of the method of the present invention, the method includes activating the auxiliary pump with a predetermined time delay after emitting the signal.
In accordance with another embodiment of the method of the present invention, the method includes regulating the dosage pump so that the suction stroke is carried out at a first speed and the discharge stroke is carried out at a second speed, the first speed being substantially greater than the second speed, and the second speed providing a substantially constant flow rate.
In accordance with another embodiment of the method of the present invention, the method includes measuring the volume of the fluid flowing through the auxiliary pump for each cycle thereof, measuring the time between each cycle of the auxiliary pump, and calculating the fluid flow through the dosage pump by determining the ratio between the measured volume of the fluid flowing through the auxiliary pump and the measured time between each cycle of the auxiliary pump.
According to the present invention, the above objects are achieved by a method and a system for monitoring a dosage pump, particularly in a dialysis machine, in which the dosage pump has a suction stroke for drawing a fluid into the dosage pump from a source of the fluid, and a discharge stroke for discharging the fluid from the dosage pump. The system includes a second pump arranged between the source of the fluid and the dosage pump, a slave chamber arranged between the dosage pump and the second pump, a level detector arranged in the slave chamber for emitting a signal when the level of the fluid in the slave chamber is below the level of the level detector, and a control arrangement for activating the second pump when the level detector emits a signal during and/or after a suction stroke of the dosage pump for refilling the slave chamber to a predetermined level before initiation of the next suction stroke.
Preferably, the control arrangement according to the present invention is arranged to fill the slave chamber with a predetermined volume above the level of the level detector. Moreover, there is preferably a regulating arrangement for regulating the dosage pump for obtaining a fast suction stroke and a regulated discharge stroke in which the discharge fluid flow rate is substantially constant.
In accordance with one embodiment of the present invention, the second pump comprises a measuring arrangement for measuring the volume of fluid passing through the second pump for each cycle, whereby the control arrangement comprises a time measurement device for measuring the time between successive cycles, and a calculation arrangement for calculating the fluid flow rate through the dosage pump by determining the ratio between said volume measurement and said time measurement.
Preferably, the second pump is a dosage pump with a known volume per revolution, or per partial revolution or the like, and the slave chamber consists of a chamber with an inlet from the second pump, which inlet is arranged immediately adjacent the side wall of the chamber.